The present invention relates to thermophotovoltaic (TPV) power generators for converting fuel to electricity using a minimum number of moving parts.
High efficiency conversion of fuel energy into electrical energy using TPV cells requires burner configurations designed specifically for TPV generation. Those designs incorporate heat exchangers, cell receiver cooling systems, infrared filters, mirrors and windows for spectral control, and special high temperature ceramic emitters. Existing units for TPV generation include high efficiency stand-alone TPV generators and appliances having configurations specifically matched to the TPV generator designs. Those units are too restrictive for immediate practical appliance retrofit applications. In typical high efficiency designs, a regenerator is required to preheat the combustion air (recovering heat from the exhaust) and to boost flame temperatures. That requirement complicates the fuel/air mixing and mandates higher pressures for the combustion gases. To obtain higher efficiencies, the TPV cell cooling system must be closely coupled to the burner/emitter/receiver combination. To meet that requirement, appliances must be inherently modified, leading to complicated designs of appliances such that existing appliances cannot be used directly. Needs exist for practical TPV power generating units that are compatible for use with existing appliances.
Existing appliance burner designs are inappropriate for adapting directly to TPV generation even at lower efficiencies. Existing appliances operate at much lower temperatures, with radiative emissions kept low to simplify burner material selection. To retrofit appliances for TPV, the entire burner unit must be replaced by new units specifically designed to attain the required high temperatures. Needs exist for universal TPV generator inserts for use in appliances which do not require modification of the appliances nor complex, complicated designs.
The present invention is a TPV generator insert for retrofitting appliances. The original burners of existing appliances are replaced by the present TPV generator inserts, thus upgrading the appliances into either cogeneration or self-powered units. Examples of appliances which may be retrofitted using the present invention include all appliances requiring hydrocarbon burner assemblies, such as residential, industrial, or commercial equipment. Those appliances include but are not limited to forced air and hydronic space heaters, water heaters, (ammonia) absorption cycle coolers, refrigerators and air conditioning systems. Immediate applications for the present invention range from simple units with several watts of electric output for ignition and controls to several hundred watt self-powered appliances requiring electrical power for fans or pumps. Self-powered units requiring at minimum a small battery for electric storage may be used for automatic controls during times when there is no TPV output. Cogeneration units are typically tied into a system including a battery storage bank, charge controller, or the like, and draw power from the system to operate their automatic controls.
Hydrocarbon fired appliances are prevalent throughout our society. In many cases those appliances are capable of generating commercially attractive electrical power outputs or sufficient electric power to render an appliance independent from frequent and unpredictable electric power outages. A hydrocarbon fired appliance retrofitted with the present TPV burner insert not only provides the appliance""s primary function, such as water heating, space heating or cooling, but also provides surplus electrical power generation as a cogenerator or independent electrical power generation as a self-powered appliance.
The present TPV burner insert is capable of retrofitting any existing hydrocarbon burner-based appliance. High electrical efficiency is not a concern, as fuel to electric conversion efficiency is less important for appliances to be converted into cogeneration or self-powered units. That lack of importance results from the higher grade xe2x80x9cwastexe2x80x9d heat, which is the primary heat source for the appliance""s operation. The total efficiency (combined electric and appliance output) is more important from a practical standpoint. By making the TPV cell cooling integral to the standard burner insert, near independence from existing appliance designs is achieved. The standard design of the present TPV burner insert is independent of the appliance to be retrofitted other than the general shape and size. Those dimensions are modified as appropriate. In the present insert, only adapters must be specific to any particular appliance. Those adapters generally include simple sheet metal and plates to mount the insert at an appropriate location in the appliance.
The present invention uses a simple, low efficiency burner in contrast to potentially higher efficiency, but more complex burners with regeneration. The maximum theoretical emitter temperatures attainable using the present insert are lower than with a regenerator, as no heat escaping in the exhaust is recaptured. The present invention avoids the high pressure requirements and problems associated with incomplete mixing. Longer burn times which tend to produce higher yields of NOx are also avoided. The high grade exhaust heat is routed and used by the appliance including the present insert.
The present burner thoroughly premixes near stoichiometric air with the fuel, maintains the mix at low temperatures to prevent preignition, and then rapidly burns the mix with high intensity just prior to forcing the ignited mix through the porous ceramic emitter. The fuel/air ratio is adjusted empirically to attain the highest temperature from the emitter, typically ranging between 1300-1500xc2x0 C.
The burner for the present TPV retrofit insert is based on the xe2x80x9cperfectly stirred reactorxe2x80x9d combustion design theory. In a simplest embodiment the burner includes a premix tube into which the fuel and air are injected at high velocities. That mixture proceeds into an expansion chamber where the mixing is completed as the gases slow down and equilibrate in pressure to give a highly flammable but cold mix. A small channel connects the expansion chamber to an insulated combustion chamber. The gases flow at high velocities from the expansion chamber to the insulated combustion chamber to prevent flashback. The gas combustion, once initiated in the combustion chamber, is very rapid and intense with short duration, briefly yielding extremely high temperatures before being forced to cool as the gas passes through the thin porous ceramic emitter and out the exhaust channel. Secondary combustion occurs beyond the insert in the appliance where adequate excess air is supplied to burn any unburned gases. The high temperature but very short duration burn of the present invention minimizes NOx generation. Since the gases are completely premixed, a uniform emitter temperature results. The entire combustion chamber is fabricated from vacuum formed high temperature alumina xe2x80x9cfoamxe2x80x9d insulation with very low thermal conductivity, low absorptivity and emissivity, and high diffuse reflectivity. The porous ceramic emitters may be fabricated from numerous types of high temperature materials. The radiation from the emitter is directed by the combustion chamber""s shape towards TPV cell receivers through a transparent high temperature quartz or sapphire heat shield or window. The radiation is further directed by infrared optical confinement mirrors and spectrally controlled by optical filters which transmit the useful portions of the radiation to the cells of the receiver and reflect the longer wavelength portion back towards the emitter. For practical applications the TPV cells of the present invention are preferably low bandgap photovoltaic cells which respond at least out to a wavelength of 1.7 micrometers.
There are three general embodiments of the present TPV retrofit burner insert assembly: a two-sided linear insert, a one-sided cylindrical insert, and a one-sided linear insert. Those embodiments differ in the geometry and symmetry of the radiation emitter/heat shield/cell path. All three embodiments exhibit common features and each will be described later in more detail. The two-sided linear insert is a simple compact unit with mirror symmetry. In one application, the two-sided linear insert is used to retrofit a small room heater and includes a linear combustion chamber having two opposing vertical flat emitters facing out towards two flat TPV circuits. If one half of that unit were to be split off in the middle along the symmetry line then curved and extended back onto itself, the one-sided cylindrical unit embodiment of the present insert is formed. The combustion chamber of the cylindrical embodiment requires a very thick insulating back wall (inside) opposing the (cylindrical) emitter which faces a circular array of TPV cells outside. That forms a circular burner insert which, in one preferred application, is used to retrofit a hot water heater, which typically has a cylindrical storage tank. The one-sided linear insert embodiment of the present invention is intermediate in design between the other two embodiments. The one-sided linear insert has a linear combustion chamber with a flat vertical emitter facing out towards a flat TPV circuit. The combustion chamber has a very thick insulating back wall opposing the emitter. That embodiment of the present burner insert has one application for retrofitting a home furnace into a self-powered unit, where the burner can only interface into one side of the furnace. With the one-sided linear insert embodiment the cell array and its cooling system are on the outside face of the furnace.
The three embodiments are generic ones, with many others being possible. Any one of those embodiments may be adapted to any appliance with proper scaling and adapters, although some are more appropriate than others. The common components of the different embodiments, including emitters, heat shields, cooling systems and components, are interchangeable between the different embodiments as needed.
These and further and other objects and features of the invention are apparent in the disclosure, which includes the above and ongoing written specification, with the claims and the drawings.